1. Field of the Invention
The present invention generally relates to a pilot signal transmission method and a mobile communication system, and more particularly relates to a pilot signal transmission method and a mobile communication system where a pilot signal for channel compensation is time-division multiplexed together with a data signal of a user, which data signal is assigned a certain bandwidth and to be wirelessly transmitted based on DFT-spread-OFDM, into a time-division multiplexed signal, the time-division multiplexed signal is frequency-division multiplexed together with time-division multiplexed signals of other users into a frequency-division multiplexed signal, and the frequency-division multiplexed signal is transmitted.
2. Description of the Related Art
In the uplink of the Evolved UTRA (enhanced system of W-CDMA) standard regarding the enhancement of the next generation mobile communication system, the use of DFT-spread-OFDM (orthogonal frequency division multiplexing) is being discussed as a wireless access technology. In DFT-spread-OFDM, a data signal of a user is assigned a certain frequency bandwidth (resource unit) and frequency-division multiplexed together with data signals of other users. DFT-spread-OFDM employs a single-carrier transmission method instead of a multi-carrier transmission method employed in, for example, OFDM and therefore provides a lower peak-to-average power ratio (PAPR). Also, DFT-spread-OFDM employs frequency-domain signal processing and therefore makes it possible to flexibly arrange components of a single-carrier signal in the frequency domain.
FIGS. 11A and 11B are drawings illustrating exemplary signal mappings in the frequency domain. In FIGS. 11A and 11B, a resource block (RB) indicates a minimum unit of frequency bandwidth assignable to a mobile station within the entire frequency bandwidth of a system. In a localized mapping shown in FIG. 11A, adjacent subcarriers (SC) are combined as an RB. In a distributed mapping shown in FIG. 11B, SCs apart from each other are combined as an RB. In either case, mobile stations use different RBs and therefore multiuser interference within a cell can be effectively avoided; in other words, frequency use efficiency is high.
In the Evolved UTRA standard, the use of the Zadoff-Chu sequence (hereafter called ZC sequence), a variation of the constant amplitude and zero auto correlation (CAZAC) sequence, for the pilot signal is being considered. When the sequence length N is an odd number, a ZC sequence is expressed by the following formula (1):
                                          c            k                    ⁡                      (            n            )                          =                              exp            ⁡                          [                                                -                                                            j2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      k                                        N                                                  ⁢                                  (                                      qn                    +                                          n                      ⁢                                                                        n                          +                          1                                                2                                                                              )                                            ]                                ⁢                                          ⁢                      (                                          n                =                0                            ,              …              ⁢                                                          ,                              N                -                1                                      )                                              (        1        )            
When the sequence length N is an even number, a ZC sequence is expressed by the following formula (2):
                                          c            k                    ⁡                      (            n            )                          =                              exp            ⁡                          [                                                -                                                            j2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      k                                        N                                                  ⁢                                  (                                      qn                    +                                                                  n                        2                                            2                                                        )                                            ]                                ⁢                                          ⁢                      (                                          n                =                0                            ,              …              ⁢                                                          ,                              N                -                1                                      )                                              (        2        )            
In the above formulas, q indicates an integer and k indicates a sequence number.
When designing a pilot signal, multicell interference must first be taken into account. In a multicell environment, instead of multiplying ZC sequences with a scramble code unique to each cell, a set of ZC sequences with low cross-correlation are generated and assigned to the pilot signals of each cell. In this case, to maximize the number of sequences k in the set of ZC sequences and thereby to maximize the flexibility in assigning ZC sequences to each cell, the sequence length N must be a prime number. Secondly, to obtain channel estimates for the frequency band assigned to a data signal, a pilot signal is preferably assigned the same bandwidth as that of the data signal.
The ZC sequence is a variation of the CAZAC sequence and therefore has constant amplitude in the time domain and the frequency domain and zero autocorrelation except when its phase difference is 0. Therefore, using ZC sequences as pilot signals makes it possible to keep the PAPR of a transmitted signal substantially low. Also, using ZC sequences as pilot signals makes it possible for a receiving end to substantially reduce the fluctuation in the SNR of channel estimates in the frequency domain between subcarriers.
When using ZC sequences as pilot signals, if the pilot signals are multiplied with a scramble code unique to each cell as in the case of data signals, the properties of the ZC sequences that are peculiar to a CAZAC sequence variation are lost. Therefore, when using ZC sequences as pilot signals for uplink transmission in a cellular system, rather than multiplying ZC sequences with a scramble code unique to each cell, it is preferable to generate multiple ZC sequences with low cross-correlation by changing the sequence number k and to assign the generated ZC sequences to each cell.
To improve the flexibility (number of replications) in assigning ZC sequences to each cell, it is preferable to be able to efficiently generate ZC sequences with low cross-correlation. It is known that when the sequence length N is a prime number, N−1 number of ZC sequences with low cross-correlation can be generated. For this reason, it is proposed to use ZC sequences having prime sequence lengths N as pilot signals for uplink transmission in a cellular system.
FIG. 12 is a drawing illustrating an exemplary mapping of pilot and data signals in the time and frequency domains and used to describe problems in a conventional technology. In FIG. 12, for brevity, it is assumed that the intervals of subcarriers for pilot signals and data signals are the same. Data signal and pilot signal regions are time-division multiplexed and one pilot signal region is provided before and after one data signal region. According to RB assignment information, RB1 and RB2 are assigned to the data signal region of mobile station A and the same frequency band as that of the data signal region is assigned to the pilot signal regions.
[Non-patent document 1] “Multiplexing Method for Orthogonal Reference Signals for E-UTRA Uplink” Agenda Item: 11.2.1, R1-061193, 3GPP TSG-RAN WG1 Meeting No. 45, Shanghai, China, 8-12 May, 2006
However, while the number of subcarriers for a data signal is an integral multiple of the number of subcarriers in an RB, the number of subcarriers (sequence length N of a ZC sequence) for a pilot signal must be a prime number as described above. Therefore, the frequency bandwidths used by a data signal and a pilot signal are basically different. Also, it is necessary to prevent interference between pilot signals of mobile stations that use adjacent RBs.
One way to solve the above problems is to set the number of subcarriers (sequence length of a ZC sequence) for a pilot signal to the largest prime number within the bandwidth of a data signal. In FIG. 12, 13 subcarriers, where 13 is the largest prime number within the frequency bandwidth 16 (unit of bandwidth is omitted here) of the data signal, are assigned to each of the pilot signals. In this case, however, the pilot signals do not cover the frequency bands corresponding to both ends of the data signal. Therefore, the channel estimates for the one rightmost subcarrier and the two leftmost subcarriers of the data signal must be extrapolated.
There are two major factors that affect the accuracy of channel estimation at a receiving unit of a base station. The first factor is thermal noise components and interference signal components contained in a received signal. The second factor is the accuracy of interpolation/extrapolation by a time and frequency interpolation/extrapolation unit. Channel distortion becomes greater in the time direction in proportion to the traveling speed of a mobile station and in the frequency direction in proportion to the delay spread. When channel distortion is low, interpolation/extrapolation can be performed accurately to a certain extent. However, since channel distortion occurs in a very complicated manner, it is difficult to accurately perform interpolation/extrapolation when channel distortion is high. Also, in urban areas, normally, channel distortion is greater and more complicated in the frequency direction than in the time direction, and an extrapolation method provides less accurate results than an interpolation method.
As described above, when a ZC sequence with a prime number sequence length is used as a pilot signal for uplink transmission, the bandwidth of the pilot signal may become different from that of a data signal, making it necessary to extrapolate channel estimates in the frequency direction. This, in turn, greatly reduces the accuracy of channel estimation and thereby degrades the reception characteristics of a data signal. On the other hand, making the bandwidth of a pilot signal larger than that of a data signal may cause interference between pilot signals of different users.